HomeMy Public PortalAboutCity of Charlack3/26/2021 City of Charlack, MO Design and Development Standards: Section 405.220 Stormwater Quality Best Management Practices.
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Ord No
Section Stormwater Quality Best Management Practices
A. A. Purpose And Intent. Two (2) consequences of construction and development are increased runo created by the changed
properties of ground surface and the rate of discharge of this increased runo . These are both of great relevance to
stormwater. The natural condition of the land before development is in relative balance with the natural capacity of the
receiving creeks. Normally the undeveloped conditions provide greater permeability and longer times of concentration. By
modi cation of the ground surface from the irregular, pervious and vegetation-covered, the areas are changed to more
impervious, more quickly drained and, in some cases, denuded of vegetation. It is the policy of the City of Charlack to
protect and promote the public health, safety and general welfare. The management of stormwater will reduce the erosion
of land and creek channels, will reduce the possibility of damage to public and private property, and will assist in the
attainment and maintenance of water quality standards, and will preserve the environmental quality of the watercourses in
the City.
B. Requirements — Creek Bank Setback. Development along natural watercourses shall have residential lot lines, commercial
or industrial improvements; parking areas and driveways set back a minimum of twenty- ve (25) feet from the top of the
existing stream bank. This is not meant to prevent access across creeks. Except as otherwise provided herein or as allowed
by the City, the section of land between a natural watercourse and lot lines shall be designated as common ground and
drainage easement to be maintained by the trustees or homeowners' association of the subdivision within all types of
residential developments. In lieu of the common ground requirement, the section of land between the natural watercourse
and twenty- ve (25) feet from the top of bank can be:
1. Private property with an easement dedicated to the trustees or homeowners' association requiring maintenance of
the areas as the responsibility of the trustees or homeowners' association,
2. Private property subject to a conservation easement in favor of an organization or land trust dedicated to
environmental protection or land preservation such as Trailnet, Inc., the Open Space Council for the St. Louis Region
or similar organization,
3. Set aside in a stream bank mitigation program approved by the United States Army Corps of Engineers under the
provisions of the Clean Water Act.
All developments shall maintain a setback minimum of fty (50) feet from the top of the existing bank to any
building structure requiring a construction permit that necessitates excavation or also requires a foundation.
Commercial and industrial areas shall have creek areas dedicated as drainage easements. Notwithstanding the
foregoing setback requirements, an applicant may obtain a variance to develop within the stream bu er area
from the City provided the applicant is able to demonstrate that the water quality and stream morphology will
not be adversely a ected or adequate mitigation is provided to o set such adverse a ects. Mitigation may
include, but not be limited to, the following:
1.Installing additional erosion and sediment control;
2.If the stream has the potential for lateral movement, installing stream stabilization practices within the
stream;
3.For those sites where vegetation does not exist, establishing vegetation; and/or
4.Additional tree planting.
C. Post Construction — Water Quality.
1. In order to preserve the quality of water in natural streams, it is important to provide a mechanism to remove
contaminants on the site prior to water entering the natural water course. Typically called Post Construction BMPs
(Best Management Practices), these methods identify a critical water quality volume that will need to receive a
treatment to remove certain contaminants. These improvements, whether structural or non-structural, will remain in
place after the construction is completed. The concepts introduced in the following paragraphs are taken from the
Georgia Stormwater Management Manual, Volume 2, August, 2001. Nothing in the following paragraphs shall change
or replace any of the City's detention ordinances.
3/26/2021 City of Charlack, MO Design and Development Standards: Section 405.220 Stormwater Quality Best Management Practices.
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Water quality volume (WQv): The water quality volume (denoted as the WQv) is the storage
needed to capture and treat the runo from ninety percent (90%) of the recorded daily rainfall
events. In numerical terms, it is equivalent to one and fourteen hundredths (1.14) inches of rainfall
multiplied by the volumetric runo coe cient (Rv) and site area. The WQv is directly related to
the amount of impervious cover created at a site. A minimum WQv of two-tenths (0.2) inches per
acre shall be met at all sites.
Redevelopment projects can appeal to the Planning and Zoning Commission to reduce or eliminate
WQv if it can be shown that there are stormwater enhancements being provided.
2. As a basis for determining water quality treatment volume, the following assumptions may be made:
a. The water quality volume WQv for o site areas is not required. The following equations are used to determine
the storage volume, WQv (in acre/feet of storage):
WQv =[(P)(Rv)(A)]/12
P =1.14 inches of rainfall
Where:WQv =water quality volume (in acre/feet)
Rv =0.05 + 0.009(I) where I is percent impervious cover
A =area in acres
b. Measuring impervious cover. The measured area of a site plan that does not have vegetative or permeable cover
shall be considered total impervious cover.
c. Multiple drainage areas. When a project contains or is divided by multiple drainage areas, the WQv volume shall
be addressed for each drainage area.
d. O site drainage areas. The WQv shall be based on the impervious cover of the proposed site. O site existing
impervious areas may be excluded from the calculation of the water quality volume requirements.
e. BMP treatment. The nal WQv shall be treated by an acceptable BMP(s) from the list presented in Subsection
B .
f. Subtraction for non-structural practices. When non-structural practices are employed in the site design, the
WQv volume can be reduced in accordance with the conditions outlined in Subsection C .
g. Extended detention for water quality volume. The water quality requirements can be met by providing a twenty-
four (24) hour drawdown of a portion of the water quality volume (WQv) in conjunction with a stormwater pond
or wetland system.
D. Acceptable Urban BMP Options. This Subsection sets forth ve (5) acceptable groups of BMPs that can be used to meet
the water quality volume (WQv) criteria. The design and selection of these BMPs should comply with the Georgia
Stormwater Management Manual as prepared by the State of Georgia.
Where the City's criteria or requirements are more stringent, then they shall govern. Adapting to local Missouri
environment and natural conditions should be expected but shall be as approved by the City or a higher authority.
1. The acceptable BMP designs are assigned into six (6) general categories for stormwater quality control (WQv):
a. BMP Group 1 — stormwater ponds.
b. BMP Group 2 — stormwater wetlands.
c. BMP Group 3 — in ltration practices.
d. BMP Group 4 — ltering practices.
e. BMP Group 5 — Open channel practices.
3/26/2021 City of Charlack, MO Design and Development Standards: Section 405.220 Stormwater Quality Best Management Practices.
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See Section 13 of the Georgia Stormwater Management Manual for additional BMP options.
Section 3 of the Georgia manual has detailed descriptions of each control.
To be considered an e ective BMP for stand-alone treatment of WQv, a design shall be capable of:
a.Capturing and treating the required water quality volume (WQv),
b.Removing 8013:(0 of the TSS.
2. A combination of BMPs and/or credits is normally required at most development sites to meet all three (3)
stormwater sizing criteria.
a. BMP Group 1 — stormwater ponds. Practices that have a combination of permanent pool, extended detention or
shallow wetland equivalent to the entire WQvs include:
(1) P-1 micropool extended detention pond.
(2) P-2 wet pond.
(3) P-3 wet extended detention pond.
(4) P-4 multiple pond system.
(5) P-5 pocket pond.
b. BMP Group 2 — stormwater wetlands. Practices that include signi cant shallow wetland areas to treat urban
stormwater but often may also incorporate small permanent pools and/or extended detention storage to achieve
the full WQv include (modi cation of existing wetland areas will require a Corps 404 permit):
(1) W-1 shallow wetland.
(2) W-2 ED shallow wetland.
(3) W-3 pond/wetland system.
(4) W-4 pocket wetland.
Wetlands shall not be used for control of water quantity (i.e., the ood protection
volume).
c. BMP Group 3 — in ltration practices. Practices that capture and temporarily store the WQv before allowing it to
in ltrate into the soil over a two (2) day period include:
(1) I-1 in ltration trench.
(2) I-2 in ltration basin.
In ltration practices will be allowed on sites where it is proven that in ltration will work.
Percolation rates shall be determined for proper use.
d. BMP Group 4 — altering practices. Practices that capture and temporarily store the WQv and pass it through a
lter bed of sand, organic matter, soil or other media are considered to be ltering practices. Filtered runo may
be collected and returned to the conveyance system. Design variants include:
(1) F-1 surface sand lter.
(2) F-2 underground sand lter.
(3) F-3 perimeter sand lter.
(4) F-4 organic lter.
3/26/2021 City of Charlack, MO Design and Development Standards: Section 405.220 Stormwater Quality Best Management Practices.
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(5) F-5 pocket sand lter.
(6) F-6 bioretention.*
(7) F-7 proprietary ltering system.
*may also be used for in ltration.
A maintenance agreement and maintenance schedule shall be required.
e. BMP Group 5 — open channel practices. Vegetated open channels that are explicitly designed to capture and
treat the full WQv within the dry or wet cells formed by check dams or other means include:
(1) dry swale.
(2) wet swale.
(3) lter strips.
Open channel practices shall be designed with the proper plantings. Wet swales shall
be designed to drain out over time.